1. Field of the Invention
The present invention is directed to a luminophore plate of the type having a substrate and an auxiliary layer lying thereabove onto which a storage luminophore layer is applied.
2. Description of the Prior Art
X-ray luminophores are generally employed in medical technology and in non-destructive materials inspection. In these applications, scintillators are utilized for spontaneous emission under X-ray stimulation, and storage luminophores are utilized for forming and storing electrons and holes for subsequent photo-stimulated emission (PSL) when irradiated with, for example, red light.
X-ray luminophores on the basis of alkali halides play a very specific role for such purposes. Examples are CSI:Na in X-ray image intensifier, CsI:TI in a-Si detectors and, recently CsBr:Eu as a storage luminophore plate as described, for example, in Proc. SPIE, Vol. 4320 (2001), “New Needle-crystalline CR Detector” by Paul J. R. Leblans et al., pages 59–67.
All of the aforementioned medical applications of alkali halides have in common the fact that the layers are produced by thermal evaporation of the alkali halides (CsBr, CsI) and of the respective dopants (TlI, NaI, EuBr2). Dependent on the vapor pressure of the materials, the substances can be evaporated from one or from two evaporator vessels, as described, for example, in German OS 100 61 743 and German PS 195 16 450.
To achieve a needle-shaped layer structure which has the ability to guide light, the vapor depositions disclosed in the above prior art publications usually is implemented at an elevated substrate temperature. The coefficient of thermal expansion of the alkali halides CsI and CsBr that are utilized lies at 4.8×10−5/° C. Glass, steel, nickel, titanium, copper and aluminum oxide ceramic can be utilized as substrates, their coefficients of thermal expansion be set forth in the following Table from D'Ans Lax, Taschenbuch für Chemiker und Physiker, Volume 1.
TABLE 1Substrate materialCoefficient of thermal expansionGlass(0.3–0.9) × 10−5/° C.Aluminum2.4 × 10−5/° C.Steel(1.0–1.8) × 10−5/° C.Nickel1.3 × 10−5/° C.Titanium0.8 × 10−5/° C.Copper1.7 × 10−5/° C.Aluminum oxide0.8 × 10−5/° C.
Shrinkage cracks arise in the luminophore layers when cooling the vapor-deposited substrates due to the lower coefficients of thermal expansion. The frequency of crack occurrence is on the order of magnitude Of 0.5–1.5 mm, as can be seen from FIG. 1 herein that shows a 50-power scanning electron microscope image of a known CsBr:Eu layer. The cracks have a width of up to approximately 10 μm, as can be seen in FIG. 2 that shows a 1000-power scanning electron microscope image of a known CsBr:Eu layer.
As is known, more light is coupled out of a luminophore layer at grain boundaries, gaps and cracks than from the luminophore needles themselves. The corona exposure according to FIG. 3 shows a microscopic illustration of an incident light point of a CsBr:Eu layer with noticeably brighter gaps and a crack that demonstrates this behavior.
The problem area that has been described is especially pronounced for storage luminophore layers such as, for example, CsBr:Eu. In the readout event, the surface of the luminophore layer is thereby scanned with a “red light spot” having a diameter of 50–150 μm. In the case of a glass substrate, however, the scanning also can ensue from the “underside”. A non-uniform readout of the stored electron-hole pairs thereby occurs corresponding to the layer structure. FIG. 4 shows the frequency-dependent quantum efficiency (DQE) of a storage luminophore layer. The “unnatural” course of the DQE curve from high to low spatial frequencies can be clearly recognized. A “forced” plateau is present in the region around 1 LP/mm. This effect is even noticeably intensified given a higher X-ray dose.
In contrast thereto, the light in the luminophore needles is generated by the X-ray quanta in the luminophore layers such as, for example, CsI:Na or CsI:Tl. The effect of a layer structure is not as problematical, especially when no photocathode is present on the luminophore needles—as in the case of an X-ray image intensifier.
In context of the aforementioned prior art, attempt has been made to generate many fine gaps around each luminophore needle by admitting gas during the vapor deposition process of the luminophore layers. It has been shown in practice, however, that this hope is achieved only conditionally, as a cathode luminescence exposure of a known CsBr:Eu layer manufactured according to German OS 100 61 743 magnified 250 times, shown in FIG. 5, demonstrates.
Another disadvantage of this evaporation method is that the density of the layer becomes lower the higher the gas pressure is during the vapor deposition. This results in the geometrical layer thickness increasing by approximately 20% for identical X-ray absorption and increased “crosstalk” of light in neighboring regions thus becomes possible. The MTF degradation is exactly as high as given a 20% thicker layer with “normal” density and correspondingly higher X-ray absorption and DQE.
German OS 29 29 745 discloses the manufacture of a luminescent screen with a grid structure. An attempt was made to prescribe the needle size with the grid structure of the substrate material by means of a designational roughening. Each “nub”—original surface of the substrate material—is followed by a “trench”, an etched-in depression. This means that mammography applications are practically not possible because of the small structural size that is required. Moreover, the structuring method is highly material-dependent because of different etching solutions, which is unfavorable in terms of fabrication technology and not environmentally compatible.